Highly bidirectional antennas allow the system designer to "point" the antenna to receive signals from a certain direction or illuminate a relatively small area. For example, in a radar-guided missile, the receiving antenna for the radar is pointed roughly toward the targeted area or object. Directivity and high efficiency see Johnson and Jasik, Antenna Engineering Handbook 17-32 (1984)) provides a much stronger signal to the reception circuitry. This increased signal strength increases the effective range and the accuracy of the radar system. Similarity, limiting the received signal to the target area minimizes the risk of jamming signals causing erroneous results from the radar system.
One method known in the prior art for improving directivity in antenna systems is the use of array antennas. Array antennas include multiple receiving elements, usually arrayed on a flat plane in a regular array. The receiving elements are coupled to receiving circuitry by equal length waveguides. Electromagnetic waves traveling in a direction normal to the plane of the antenna array will constructively interfere because they arrive at the receiving elements in phase. On the other hand, waves traveling off of the normal will arrive at the receiving elements at slightly different times. Thus, the signals will be mixed at the coupler out of phase. The degree to which the signals are out of phase may be determined geometrically using the angle of incidence and the wavelength of the received signal. When the signals are one-half wavelength out of phase, the signals will destructively interfere. For angles of incidence between the normal and destructive interference, the signal level at the coupler diminishes from the normal level to the destructive interference level. At an angle farther off the normal from the destructive interference level, the distance induced delay reaches a full wavelength and the signals once against constructively interfere. Such signal interference creates an off normal area of sensitivity known as a side lobe. Side lobes reduce the directivity of the antenna and are thus undesirable.
One technique known in the art for reducing the effect of side lobes is the image plate concept. The image plate idea was originally set forth by Von Trentini, "Partially Reflecting Sheet Arrays," IRE Transactions on Antennas and Propagation, October 1956 pp. 666-71. An example of an image plate structure is disclosed in Ballee et al, U.S. Pat. No. 3,990,078.
In an image plate antenna, a partially reflecting sheet of material is placed parallel to the plane of the radiating element or the element. The plane itself is composed of a reflective material. The spacing between the sheet and the plane is one-half wavelength. A wave entering the antenna normal to the plane will be reflected by the plane and then re-reflected off the partially reflective sheet. The wave travels one full wavelength and thus is in phase with the incoming wave. This creates a damped (because the sheet is partially reflective ) resonant cavity. Because of the phase reversal of a wave upon reflection, the receiving elements must be half way between the sheet and the plate for reflected, direct and re-reflected waves to reach the receiving element in phase. See Immell and Sasser, "A Highly Thinned Array Using the Image Element Antenna," Third Annual Antenna Applications Symposium, Motorola Government Electronics Division, September 1979. Waves which enter the antenna off of the normal travel a distance greater than one wavelength depending upon the angle of incidence. Thus, under such circumstances, less than perfect constructive interference occurs, and leads toward destructive interference as the incident angle approaches an angle 60.degree. off of the normal.
The lateral (parallel to the plane) reflection creates a "second source" effect. Because the reflected signals are combined at the array elements along with other reflected waves and direct waves, the signal received by the element is analogous to the signal combined at the coupling of the waveguides, with analogous effects, i.e., increased directivity. However, the image plate concept relies on two competing properties of the partially reflective sheet. On the one hand, the sheet must pass the wave to allow it to be received by the elements at all. On the other hand, the sheet must reflect as much as possible to achieve the resonance effect. Usually dielectric partially reflective sheets allow 70 to 90 percent of the wave to pass and 10 to 30 percent is reflected. With such a small percentage of reflected energy, the image plate effect is negligible after one full reflection.
Another technique for reducing the effect of side lobes is the short backfire technique. An example of the short backfire technique may be found in Ehrenspeck, U.S. Pat. No. 3,742,513. The short backfire antenna is composed of at least one element on a reflective plane. The element is positioned a quarter wavelength above the plane and a reflective plate is positioned directly a quarter wavelength above the element and thus one half wavelength above the plane. Waves entering the antenna at near the normal will establish resonant waves between the plate and the plane, thus enhancing the reception or transmission of normal waves. Incoming waves off of the normal will constructively or destructively interfere in a manner similar to that described for the image plate antenna.
Both the image plate antenna and the short backfire antenna operate, in part, on a spatial dispersion concept that generates "phantom" elements so the antenna behaves as if it had more elements. However, the spatial dispersion from the source element of these phantom elements is limited. This limitation becomes a problem in systems where electro-optic devices must be integrated with the antenna. Because of the space they occupy, the electro-optic devices cause an aperture blockage of the antenna. Aperture blockage causes gain loss and increases side lobes (see Johnson and Jasik, Antenna Engineering Handbook, p. 17-32 (1984)). It is a object of the present invention to provide a system which provides a high-gain antenna with minimized side lobes and whichis compatible with the inclusion of electro-optic devices or some other antenna type such as a four-arm monopulse spiral or millimeter-wave homing antenna system.